<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Undefined</title>
</head>
<body>
</body>
<script>
    let age;

    // 控制台打印，可以传入多个变量使用逗号隔开，然后控制台中会自动进行拼接
    console.log("age =", age); // age = undefined

    console.log("sex =", sex); // 报错 sex is not defined
</script>
</html>